The present invention relates to a light-scattering sheet (film) for liquid crystal devices useful for the high-luminance display of images, a method of producing the sheet, a light-scattering composite sheet utilizing said sheet, a liquid crystal display device, and a method of producing said light-scattering composite sheet.
The liquid crystal display (LCD) device is utilized broadly in the display segments of personal computers, word processors, LC televisions, chronometers, desktop calculators and other electrical and electronic products. Since the liquid crystal by itself does not emit light, a backlight for illuminating the liquid crystal cell from the back side is used in liquid crystal display devices to the exclusion of low-luminance applications such as watches and desktop calculators.
Recent years have witnessed advances in the construction of infrastructures for telecommunications systems such as internets and in the network consolidation of information through the computer-communications equipment integration. By network consolidation, the access to information is freed from restrictions as to time and place. For an efficient utilization of such networks, portable information terminals such as PDA (personal digital assistance) have recently been developed. Moreover, in lieu of notebook-sized personal computers, further-downsized mobile personal computers of reduced thickness and weight are under development.
Since portability is required of these devices, it is necessary to reconcile the need for a longer battery operating time with the need for reduced thickness and downsizing of communications devices. Therefore, display devices for use in such portable telecommunications equipment must be of reduced thickness, reduced weight and low power consumption. Particularly for attaining the low power consumption goal, a technology for brightening the display screen by exploiting natural light has been sought for replacing the conventional technology using a backlight. The display mode which is considered to be most promising is a reflecting liquid crystal display device. Particularly, to keep abreast with the increasing versatility of data accompanying the ever-continuing advances in multimedia, there is a demand for reflecting-mode liquid crystal display devices not only capable of color display and high image-quality (high-definition) display but also of low production cost.
As the reflecting liquid crystal display device, there is known a variety of devices such as TN (twisted nematic) and STN (super twisted nematic) devices but for color display and high-definition display, devices utilizing a polarizer (one polarizer plate type) is preferred. For example, the R-OCB mode in which the liquid crystal layer is of the HAN (hybrid aligned nematic) alignment has excellent characteristics such as low voltage, wider viewing angle, high-speed response, middle color rendition and high contrast. As the display device capable of forming a high-definition image on the screen, active matrix type liquid crystal display devices such as TFT (thin film transistor) which control all pixels each independently are also generally available. However, TFT and other active matrix type liquid crystal display devices must be fabricated using glass substrates because hundreds of thousands or more transistors must be formed on a substrate. In contrast, STN (super twisted nematic) liquid crystal display devices which perform matrix image displays using bar electrodes are inexpensive as compared with TFT devices and, in addition, plastic substrates can be used as the substrates for electrodes (support plates) and reflecting-mode plastic LCDs can be fabricated.
In the reflecting type LCD, the brightness of the screen is insured in such a manner that the light incident on the liquid crystal layer (natural light, ambient light) is efficiently taken in and reflected with a reflector and the reflected light is appropriately scattered (for the prevention of total reflection). When a sufficient brightness level cannot be obtained, depending on the environment of use, even if the natural or ambient light is fully exploited, a front light is at times used for supplying light from the side of the display screen of the LCD. As the reflector, a light-reflecting back electrode system utilizing light reflective electrodes and a laminate sheet such that a reflecting film has been laminated on the surface of the electrode-carrying substrate can be employed. For example, in Japanese Patent Application Laid-Open No. 22887/1998 (JP-63-22887A) and Photofabrication Symposium ""92 sponsored by the Japanese Society of Printing, the fundamental technology about reflecting type LCD and the liquid crystal device given an enlarged viewing angle through the prevention of total reflection by means of a surface-corrugated metal thin film as the back electrode (lower electrode) were introduced. Such a reflecting liquid crystal display device is designed to scatter light for avoiding specular reflection and for this purpose, it includes a reflector (or a light-reflecting back electrode) whose surface has been appropriately roughened. However, it requires an elaborate processing technique and a high cost. Moreover, when the display is to be a color display, a color filter is used in addition to said polarizer. In case a color filter is used, the proportion of loss of reflected light is increased and the above diffusion plate system cannot impart enough brightness to the display screen. In the color system, it is particularly important to impart high luminance by directing diffused light in a given direction (directed diffusion). In order to increase this directionality with the diffusion-reflector system, it is necessary to precisely control the geometry and distribution pattern of reflector surface irregularities but this is a costly procedure.
For the purpose of insuring a high luminance by scattering reflected light, there has been disclosed a technology in which, in lieu of a light-diffusing reflector, the liquid crystal layer is constituted as a dispersed structure such that the liquid crystal and a high polymer are interdispersed (Japanese Patent Application Laid-Open No. 258624/1994). Also known is a liquid crystal display device in which a transmitting type light-scattering sheet is used in lieu of a diffuser plate.
For example, a technology comprising forming a light-scattering transparent resin layer either internally or externally of a liquid crystal cell is known. As an example of the display device having a light-scattering layer within the liquid crystal cell, Japanese Patent Application Laid-Open No. 98452/1995 (JP-7-98452A) discloses a display device comprising a transparent resin layer (light-scattering layer) containing a dispersion of fine particles as interposed between the electrode and substrate sheet (electrode-supporting substrate) of the electrode plate. Moreover, Japanese Patent Application Laid-Open No. 318926/1995 (JP-7-318926A) discloses a display device comprising a support plate carrying a transparent electrode, a liquid crystal layer, and as interposed therebetween, a diffusion layer comprising randomly oriented liquid crystalline polymer molecules. Meanwhile, Japanese Patent Application Laid-Open No. 261171/1995 (JP-7-261171A) discloses a display device having a light diffusing layer externally of a liquid cell, specifically a display device comprising a polarizer film on the outer surface of an electrode plate and, as formed on the surface of said polarizer film, a light-scattering layer comprising a phase-separated dispersion of two or more kinds of resins varying in refractive index. Japanese Patent Application Laid-Open No. 8430/1986 (JP-61-8430B) discloses a liquid crystal display device comprising a polarizer layer formed on the front side of a liquid crystal cell and, as formed thereon, a light-scattering layer. However, the polarizer plate usually has a highly sophisticated surface hardness characteristic and an adequate anti-glare characteristic. Therefore, when a light-scattering layer is formed on the surface of such a polarizer plate, the surface of the display device (that is the light-scattering sheet) becomes vulnerable to marring so that the visibility of the screen of the reflecting type liquid crystal display apparatus is lowered to adversely affect the image quality. In particular, it becomes difficult to maintain the image quality over a long time. Furthermore, because a double image, namely the liquid crystal image and the image from the light-scattering layer is formed, the sharpness of the image is sacrificed (blurred image) to adversely affect the image quality. Meanwhile, a resin sheet as polymerized by utilizing holography for imparting directionality to a transmitting type light-scattering sheet is known (The synopsis of Lectures at Japanese Society of Liquid Crystal Science, 1998) but the production process is complicated and costly.
As disclosed in Japanese Patent Application Laid-Open No. 27904/1995 (JP-7-27904A) and Japanese Patent Application Laid-Open No. 113902/1997 (JP-9-113902B), there also are known transmitting type liquid crystal display units such that a particle-dispersion sheet having an islands-in-an ocean structure comprised of plastic beads and a transparent resin matrix is interposed between a back light and a liquid crystal cell.
Under the circumstances, the object of the present invention is to provide a light-scattering sheet (or film) conducive to a liquid crystal display of high image quality, a light-scattering composite sheet (or film), a liquid crystal display device, and a method of producing said light-scattering sheets.
It is a further object of the present invention to provide a light-scattering sheet (or film) capable of imparting diffusibility and directionality to reflected light, a light-scattering composite sheet (or film), a liquid crystal display device, and a method of producing said light-scattering sheets.
It is another object of the present invention to provide a light-scattering composite sheet (or film) useful for the manufacture of a high-luminance, high definition liquid crystal display device at low cost and a liquid crystal device utilizing said composite sheet (or film).
It is a further object of the present invention to provide a liquid crystal display device insuring a sustained image quality over a long period of time.
It is yet another object of the present invention to provide a method by which a directionally diffusing sheet (or film) can be produced with ease.
The inventors of the present invention made intensive investigations to accomplish the above objects and found that subjecting a plurality of resins varying in refractive index to spinodal decomposition gives an isotropic bicontinuous phase structure with ease and that by using a sheet having such a bicontinuous phase structure, a high order of directionality can be imparted to diffused light. The inventors further discovered that with a composite sheet composed of a light-scattering sheet and a polarizer sheet, an optical retardation film, a reflector or a transparent electrode layer, not only an image of high quality can be produced but also a liquid crystal display device can be manufactured easily and at low cost. It was further discovered that when the light-scattering sheet is disposed in a defined position in a reflecting type liquid crystal device, the durability of the reflecting type liquid crystal display device can be improved and, in addition, an image of high precision can be obtained. The present invention has been developed on the basis of the above findings.
The light-scattering sheet of the present invention, therefore, comprises a light-scattering layer having an isotropic bicontinuous phase structure comprised of a plurality of polymers varying in refractive index. The average interphase distance of this bicontinuous phase structure may for example be about 1 to 20 xcexcm and the difference in refractive index between the component polymers may for example be about 0.01 to 0.2. The plurality of polymers may have a lower critical solution temperature (LCST) type phase separation mode. The critical solution temperature of the composition comprising said plurality of polymers may for example be about 50 to 300xc2x0 C. The average molecular weight of each component polymer may for example be about 10,000 to 300,000 and the polymer may for example be a styrenic resin, (meth)acrylic resin, avinyl ether resin, a halogen-containing resin, a polycarbonate resin, a polyester resin, a polyamide resin, a silicone resin, a cellulose derivative, or a rubber or elastomer. The light-scattering sheet of the present invention has a transmittance value of 70 to 100% and is capable of diffusing incident light isotropically. The diffused light has a maximum intensity distribution at a diffusion angle of 3 to 60xc2x0. For example, when the intensity of light transmitted through the light-scattering sheet is plotted against diffusion angle (xcex8), the ratio of the intensity I (xcex80) of linearly transmitted light to the maximum intensity I (xcex8max) of diffusedly transmitted light, i.e. [I (xcex80)/I (xcex8max)], may be about 3000/1 to 1/1.
The light-scattering composite sheet according to the present invention consists of a light-scattering sheet comprising a light-scattering layer and, as formed at least on one side of said light-scattering sheet, at least one member selected from the group consisting of a polarizer, an optical retardation sheet, a reflector and a transparent electrode layer. The light-scattering layer has a phase separation structure composed of a plurality of solid components varying in refractive index, with the difference in refractive index being about 0.01 to 0.2. The light-scattering layer may have a particle dispersion structure comprised of a transparent matrix or base resin and, as dispersed therein, a fine powder having a different refractive index, or may have an isotropic bicontinuous phase structure.
The liquid crystal display device of the present invention includes a light-scattering sheet comprising a light-scattering layer. In a first embodiment, said light-scattering sheet is disposed in a defined position in a reflecting type liquid crystal display device. Thus, this reflecting type LCD device comprises a liquid crystal cell defined by a juxtaposed set of a transparent front electrode plate comprising a transparent electrode layer and a substrate sheet supporting said conductive layer, a back electrode plate comprising a conductive layer and a substrate sheet supporting said conductive layer, and a liquid crystal hermetically interposed between the conductive layers of said two electrode plates, and as disposed forwardly of said liquid cell, a polarizer. The light-scattering sheet is disposed in at least one of the following positions or modes (i) to (iii).
(i) The light-scattering sheet interposed between the polarizer and the front electrode plate.
(ii) The light-scattering sheet interposed between the back electrode plate and the reflector disposed behind said back electrode plate.
(iii) The light-scattering sheet functioning as a substrate sheet.
In a second embodiment, the liquid crystal display device according to the present invention includes a light-scattering sheet having an isotropic bicontinuous phase structure comprised of a plurality of polymers varying in refractive index.
The present invention is further directed to a method of producing a light-scattering sheet which comprises molding a composition comprising a plurality of polymers varying in refractive index into a sheet and subjecting the sheet to spinodal decomposition to form an isotropic bicontinuous phase structure.
As used in this specification, the term xe2x80x9csheetxe2x80x9d means any two-dimensional product regardless of its thickness, thus inclusive of film.
The term xe2x80x9clight-scattering sheet having a bicontinuous phase structurexe2x80x9d is used herein to include a light-scattering sheet having an intermediate structure between said bicontinuous phase structure and a heterogeneous phase (islands or droplets) structure.